1. Field of the Invention
The present invention relates to an imaging and synthesizing method for a wide dynamic range and an imaging device for a wide dynamic range, for imaging an object having a light intensity of wide dynamic range in the natural world, without resulting in white saturation or black level cutoff. "White saturation", as used herein, refers to the phenomena in which an increase in the signal level output from the imaging element does not result in any appreciable increase in the light intensity. "Black level cutoff", on the other hand, is used herein to refer to the phenomena in which a reduction in the signal level output from the imaging element does not result in an appreciable decrease in the light intensity.
2. Description of the Related Art
In a conventional technique such as disclosed in Japanese Laid-Open Patent Publication No. 63-306779, the exposure amount (i.e., a charge accumulation period) of an imaging device is controlled, so as to successively synthesize images, thereby accommodating the imaging device to a moving picture. FIG. 17 shows a construction of an imaging device for implementing the conventional technique. The imaging device shown in FIG. 17 includes an imaging element (CCD) 205, a signal processing circuit 204, an A to D converter 201, a control circuit 202, a field memory 206, a switching circuit 208, a field memory 207, and a D to A converter 203. The concept of the conventional imaging device for a wide dynamic range is briefly described with reference to FIG. 18.
As shown in (1) of FIG. 18, the imaging is sequentially performed while the charge accumulation period of the imaging device (CCD) 205 is switched between 1/000 sec. and 1/120 sec. An image obtained for the charge accumulation period of 1/1000 sec. is temporarily stored in the field memory 206. At the timing of an image obtained for the charge accumulation period of 1/120 sec., the stored image which has been obtained for the charge accumulation period of 1/1000 sec. is read out from the field memory 206. The period of 1/120 sec. and the period of 1/1000 sec. are switched (synthesized) depending on the conditions, and the resulting image is written into the field memory 207 ((2) of FIG. 18). Although the switching condition described in the specification includes an inconsistency, it is presumed as follows. If the switching condition that the white saturation occurs in the image obtained for the charge accumulation period of 1/120 sec. and that the black level cutoff is not caused in the image obtained for the charge accumulation period of 1/1000 sec. is satisfied, an image obtained for the charge accumulation period of 1/120 sec. is switched to an image obtained for the charge accumulation period of 1/1000 sec. In this way, the control circuit 202 performs the switching and synthesizing operation of the images ((3) of FIG. 18). For the synthesized image signal, the speed thereof is changed in the field memory 207, so that the scanning speed is changed so as to be the speed of a usual TV signal. Then, the synthesized image signal is output ((4) of FIG. 18).
By synthesizing the images according to the above method, an image which may conventionally include white saturation or black level cutoff is replaced with an image obtained for a different charge accumulation period, so that the dynamic range of the image can be widened.
However, by the method shown in FIG. 17, the output images are constituted by extracting images without resulting in white saturation and black level cutoff. Accordingly, if the output images exhibit high contrast and the signal levels thereof continuously change, the output images may include discontinuity between the extracted images. Thus, the image quality is deteriorated.
If the object is moving, the periods for photoelectric conversion of images are not successive, so that a single object may be observed as plural separate objects in the synthesized images.